The invention is based on a fuel injection pump as generally defined hereinafter. Slide-controlled pumps are primarily used for high-capacity output at high pressures, that is, for relatively large engines operating at slower speeds. Accordingly, deviations from the set-point injection values not only lead to poorer engine operation, but may very easily cause expensive engine damage. This danger exists whenever a failure of the control slide actuating device moves the control slide into an extreme position, in which either the injection pump injects an excessive fuel quantity, causing the engine to race, or else the onset or end of supply, determined in these pumps by the position of the control slide, causes the fuel to be injected into the engine cylinder too early or too late, which as is well known can cause thermal or mechanical overloading of the engine. Thermal overloading, in particular, causes a loss in power.
In a known fuel injection pump of the above general type, U.S. Pat. No. 3,712,763, the connecting conduit is embodied as a bore in the pump cylinder liner; it discharges into a suction chamber, surrounding this cylinder liner, of the injection pump and is blocked by the pump piston after a pre-stroke has been executed, after which the pumping of fuel to the engine can begin. Toward the end of the intake stroke and at bottom dead center of the pump piston, the pump work chamber is filled with fuel via this bore, while during the compression stroke of the pump the position of the bore determines the supply onset. The end of supply, and hence the supply quantity, is determined by the position of the control slide at that time. The opening of a quantity control conduit, which extends in the pump piston, emerges from the inner bore of the control slide to establish the end of supply. The farther the control slide is displaced, on the pump piston, toward the pump work chamber, the later will the injection be interrupted and thus the larger is the supply quantity. Hence in an extreme position of the control slide, the pump piston continues to up until reaching, or approaching close to, its top dead center position. In the drive mechanism for driving the pump piston of these slide-controlled pumps, a roller of a roller tappet, coupled with the pump piston, rolls off a cam track of a drive cam; during the high-pressure pumping, the roller rolls off on not only the cam track having a virtually straight cross section, but also on curved tracks adjoining the straight one. Within these curved tracks the cyclical pressures between the roller and the track are considerably higher. That is, while in the straight portion of the track a straight line is facing the roller circle, when the curved cam track is being traversed two circular tracks face one another, and depending on the elasticity of the material the resultant linear contact surface is considerably narrower than if a straight track and a roller are facing one another. At high pressures, which are usual with these pumps, this can cause an overload on the material and thus may destroy the drive mechanism of the injection pump. Quite aside from this, the use of this curved portion of the drive cam for high-pressure pumping also has disadvantages for the injection characteristic, because in this range the supply quantity varies greatly per angle of rotation of the camshaft and decreases down to zero. This decrease does not occur in the normal rpm range, such as the partial-load range; instead, the quantity is determined by a sharply-defined diversion effected by the emergence of the opening of the quantity control conduit from the control slide. In the threshold range described, however, the quality of the fuel injection worsens toward the end of the pump piston stroke, with all the attendant disadvantages in terms of engine operation.
In another known slide-controlled fuel injection pump, although one lacking a connecting conduit in the pump cylinder (U.S. Pat. No. 2,147,390), the onset and end of supply are determined by the axial position of the control slide, but contrarily the supply quantity is determined by rotating the pump piston; to this end, in the usual manner, an oblique control edge located in either the control slide or the pump piston cooperates with a bore provided in the opposing part. In this pump as well, there is the danger that in extreme positions of the control slide damage to the engine or to the pump drive mechanism as described above, can occur, or that the engine operation will be associated with the abovementioned disadvantages.